High-efficiency tuned inverter circuit

ABSTRACT

A simple single-ended inverter circuit efficiently converts a DC input voltage to an AC output voltage. The circuit, which functions as a self-contained Class C oscillator, comprises an energy-storing inductor and an energy-storing tank capacitor in circuit with a saturable core feedback transformer operable to provide intermittent feedback current to effect periodic conduction of a single power transistor. When this transistor is non-conductive, resonant interchange of energy occurs between the inductor and tank capacitor; periodic energy transfer from the inductor both charges the capacitor and also supplies a load, and energy transfer from the capacitor to the inductor provides reset current to the saturable transformer.

BACKGROUND OF THE INVENTION

The present invention relates in general to electrical energy conversiondevices and, more particularly, to electrical inverter circuits forconverting DC voltage into AC voltage.

DESCRIPTION OF THE PRIOR ART

As is known in the art, inverter circuits are employed to convert a DCinput voltage into an AC output voltage. It is also known that Class Coscillator circuits are useful energy conversion devices because oftheir relatively high efficiencies. However, applicant is not aware ofany prior art self-oscillating Class C oscillator containing a singletransistor to achieve high-efficiency DC to AC energy conversion.

SUMMARY OF THE INVENTION

The present invention provides a relatively simple and cost-effectivehigh-efficiency single-ended inverter circuit operable to convert a DCinput voltage, typically on the order of 12 volts, into an AC outputvoltage of several hundred volts in magnitude. The present inventioncomprises an energy-storing inductor and an energy-storing tankcapacitor in circuit with a saturable core feedback transformer operableto provide intermittent feedback current to effect periodic conductionof a single power transistor. When this transistor is non-conductive,resonant interchange of energy occurs between the inductor and tankcapacitor; periodic energy transfer from the inductor charges thecapacitor and also supplies a load, and periodic energy transfer fromthe capacitor to the inductor provides a reset current to the saturabletransformer.

It is therefore an important object of the present invention to providea DC to AC energy conversion circuit which is relatively simple indesign and inexpensive to manufacture.

Another important object of the present invention is to provide aself-contained or self-oscillating Class C oscillator circuit operableto achieve highly efficient DC to AC energy conversion.

Yet another important object of the present invention is to provide ahigh-efficiency single-ended inverter circuit comprising one transistorto which intermittent feedback current is supplied to effect periodictransistor conduction.

These and other objects of the present invention will become apparentfrom the following description which, when taken in conjunction with theaccompanying drawings, discloses a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a preferred embodiment of the invertercircuit of the present invention;

FIGS. 2(a), (b) and (c) illustrate typical waveforms of the tankcapacitor voltage, the inductor current and the transistor base voltagerespectively, for the inverter circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, an inverter circuit generally designated byreference numeral 10 is connected to a unidirectional voltage input,typically on the order of 12 volts, provided by a battery 11 to positiveand negative leads 12, 13, respectively. This DC voltage is convertedinto an alternating voltage, typically on the order of several hundredvolts, at an output taken across terminals 14, 15. Rectified linevoltage may be provided as an alternative to the battery 11. The circuit10 functions as a Class C oscillator.

Considering first its structural components, the inverter circuit 10comprises an energy-storing inductor means such as a transformer winding16 having one end thereof connected to positive lead 12 through a switch17 and having another end thereof connected to a primary winding 18 of aknown saturable core transformer means 19. The other end of the primarywinding 18 is connected to an output lead 21 which is, in turn,connected to output terminal 14. The negative battery lead 13 isconnected directly to the other AC ouptut terminal 15.

The saturable core transformer 19 has a secondary winding 22 having oneend thereof directly connected to lead 13 and another end connected to abase or control element 23 of a power transistor means 24 in order toprovide intermittent feedback current to the base when positive inductorcurrent, taken in the direction of the arrow designated I_(L) adjacentinductor 16, is flowing. Transistor 24 has a collector 25 connected tothe cathode of a rectifier or diode means 26, the anode thereof beingconnected to the output lead 21. Transistor 24 has an emitter 27connected directly to the negative battery lead 13. A diode 28 has ananode connected to lead 13 and a cathode connected to the base terminal23 of the transistor 24 for a purpose to be described.

An energy-storing tank capacitor means 29 is connected across theterminals 14, 15 and in parallel with the series-connected transistor 24and rectifier 26. As will be explained, when transistor 24 isnon-conductive, resonant interchange of energy occurs between theinductor 16 and the tank capacitor 29. A coupling capacitor 31 having avalue much smaller than that of the tank capacitor 29 connects theoutput terminal 14 to a load 32, which may be resistive, capacitive orinductive. For example, the load 32 may comprise a ballast for afluorescent lamp or a capacitive electro-luminescent panel.

Finally, a clamping circuit, utilized for a purpose to be described,comprises a diode 33 in series with another winding 34 on transformer 19and a winding 35 tightly coupled to winding 16. Diode 33 is connected tothe switched positive output of battery 11 and winding 35 is connectedto the ground lead 13. Windings 16 and 35 preferably comprise a bifilartransformer.

The operation of the circuit of FIG. 1 is best understood withadditional reference to FIGS. 2(a), (b) and (c), which illustratetypical waveforms of the capacitor 29 voltage V_(C), the inductor 16current I_(L) and the base 23 voltage V_(B), respectively, which aretaken as positive in the directions denoted by the conventional arrowsin FIG. 1.

Upon closing of the switch 17, the circuit begins to oscillate. Ifrectified line voltage is substituted for the battery 11, a known Diacstarting circuit (not shown) will be required to initiate oscillation.Considering the time period just prior to T₁, when the transistor 24 isconductive, it will remain in this condition as long as positivefeedback current is supplied to its base 23 by means of the secondarywinding 22 of the saturable core transformer 19. At time T₁, thesaturable core transformer 19 becomes saturated and the positivefeedback supplied by its secondary winding 22 ceases, causing thetransistor 24 to become non-conductive.

During the period between T₁ and T₂, energy stored in the inductor 16 asa result of positive current flowing through it is transferred to theresonant tank capacitor 29 and also to the load 32 through capacitor 31,with the result that capacitor 29 begins to charge; this flow of currentwill continue until all of the energy stored in the inductor 16 has beendischarged, which occurs at time T₂.

At the time T₂, the voltage V_(C) reaches a value much higher than thatof the B+ voltage of the battery and the tank capacitor 29 begins todischarge its stored energy back into the inductor 16, causing a reversecurrent to flow through the inductor and the series-connected primarywinding 18 of the saturable core transformer 19. This reverse currentcauses the magnetic core of the saturable core transformer 19 to bereset, thereby making the transformer ready for a new forward feedbackcycle.

This reverse current, will continue to flow until time T₄, wheninductive energy has again been transferred to the capacitor 29. At thispoint, the capacitor 29 voltage V_(C) reaches its maximum negativevalue. After T₄ positive current I_(L) starts flowing through theinductor 16. As a result, the feedback transformer 19 will now supplyfeedback currnt to the base 23 of transistor 24, causing it to becomeconductive.

The voltage V_(B) on the base 23 of transistor 24 is illustrated in FIG.2(c). The diode 28 prevents the base voltage V_(B) from exceeding thebreakdown level when the core of the saturable core transformer 19 isbeing reset.

The purpose of the rectifier 26 is to permit the voltage V_(C) to reachnegative values. It will be appreciated that this operation is moreadvantageous than that of typical prior art Class C oscillator circuitswhich do not permit instantaneous negative output voltage. If the loadrequirements are such that a negative V_(C) is not necessary, thisrectifier may be eliminated.

It is important that the saturable core transformer 19 remainunsaturated long enough for V_(C) to reach a slightly positive value,which occurs at time T₅. This positive voltage is equal to the forwarddrop of diode 26 plus the collector-emitter voltage of transistor 24. Infact, to insure cyclic energy addition to the oscillating circuit, thesaturable core transformer 19 should remain unsaturated for a short timeperiod, that is, until time T₆. Of course, the longer transformer 19remains unsaturated, the more energy is supplied to the oscillatorcircuit 10 during each cycle. Therefore, the output power of theoscillator 10 can be adjusted by varying the number of turns on thesecondary 22 of the transformer 19.

The clamping circuit comprises diode 33 and windings 34 and 35 willprevent V_(C) from rising to excessive levels under no-load conditions,the clamping energy being returned to the battery power supply 11.

It is thought that the present invention and many of its attendantadvantages will become apparent from the foregoing description and it isapparent that various changes may be made in the form, construction andarrangement of its component parts without departing from the spirit andscope of the invention or sacrificing any of its material advantages,the form described being merely a preferred embodiment of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a single-ended ClassC oscillator having a tuned L-C tank circuit, the improvementcomprising:power transistor means having a control element; andsaturable core feedback transformer means for supplying intermittentfeedback signals to the control element of the transistor means in orderto effect periodic transistor means conduction.
 2. The oscillator ofclaim 1 wherein the feedback transformer means comprises a currenttransformer.
 3. An inverter circuit connected to a unidirectionalvoltage at an input and being operable to provide an alternating voltageat an output, comprising:energy-storing inductor means connected to thevoltage input; power transistor means having a control element;saturable core feedback transformer means having a primary winding incircuit with the inductor means and the transistor means and having asecondary winding for providing intermittent feedback current to thecontrol element of the transistor means in order to effect periodictransistor means conduction; and energy-storing tank capacitor means incircuit with the power transistor means and the output for interchangeof energy with the inductor means when the transistor means innon-conductive, discharge of the capacitor means serving to provideperiodic reset current to the saturable transformer means.
 4. Theinverter circuit of claim 3 and rectifier means in series with thetransistor means.
 5. The inverter circuit of claim 3 and diode meansconnected to the control element of the transistor means in order tolimit the maximum control element voltage to a predetermined level. 6.An inverter circuit connected to a DC voltage at an input and beingoperable to provide an AC voltage at an output connected to a loadcomprising:energy-storing inductor means connected to the voltage input;power transistor means having a base, an emitter and a collector;saturable core feedback transformer means having a primary winding inseries with the inductor means and the transistor means and having asecondary winding connected in circuit with the base-emitter junctionthereof in order to provide intermittent feedback current thereto inorder to effect periodic transistor means conduction; and energy-storingtank capacitor means connected in parallel with the power transistormeans and the output for interchange of energy with the inductor meanswhen the transistor means is non-conductive; whereby periodic energytransfer from the inductor means serves to charge the capacitor meansand supply energy to the load and periodic energy transfer from thecapacitor means to the inductor means serves to provide reset current tothe saturable transformer means.
 7. The inverter circuit of claim 6 andrectifier means in series with the transistor means.
 8. The invertercircuit of claim 6 and diode means connected across the base-emitterjunction of the transistor means in order to limit the control elementvoltage to a predetermined level.
 9. The inverter circuit of claim 6 anda capacitor for coupling the load to the inverter output.
 10. Theinverter circuit of claim 6 and clamping means in circuit with theinverter input for limiting the value of the instantaneous positiveoutput voltage.
 11. The inverter circuit of claim 10 wherein theclamping means comprises a transformer winding tightly coupled to theinductor means.
 12. The inverter circuit of claim 10 wherein theclamping means comprises a clamping transformer winding tightly coupledto the inductor means and further comprises an additional winding on thefeedback transformer means in circuit with the clamping transformerwinding and the inverter circuit input.
 13. An inverter circuitconnected to a DC voltage input and being operable to provide asegmented sine wave voltage output, comprising:at least one powertransistor means comprising a base, an emitter and a collector; tankcircuit means in circuit with the transistor means and the output andcomprising an inductor and a capacitor operable to interchange energywhen the transistor means is non-conductive; and means for providingdrive current to the transistor means to effect periodic conductionthereof, each conduction period being initiated when theemitter-collector voltage of the transistor means is substantially zeroin magnitude.
 14. The inverter circuit of claim 13 wherein the drivecurrent means is operable to effect transistor means conduction onceduring each full cycle of the voltage output. .Iadd.
 15. An invertercircuit adapted to provide an AC output voltage andcomprising:semiconductor switching means connected with a source of DCvoltage as well as with a tank circuit means, said switching meanshaving control terminals and output terminals, said control terminalsbeing provided with a signal operative periodically and alternatingly torender said switching means in an ON-state and in an OFF-state, saidON-state being characterized by an ON-time during which the switchingmeans exhibits a relatively low resistance to the flow of electriccurrent, said OFF-state being characterized by an OFF-time during whichsaid switching means exhibits a relatively high resistance to the flowof electric current, said tank circuit means: (i) having an inductor anda capacitor operable to resonantly interact, (ii) having a naturalresonance frequency, (iii) being operative to cause the AC outputvoltage to be, at least in part, sinusoidal in waveshape, independent offrequency, and (iv) being connected with said output terminals; andsaturable reactor means connected in circuit between said outputterminals and said control terminals and operative: (i) to provide saidsignal, and (ii) by way of its saturation characteristics, to determinethe duration of said ON-time. .Iaddend.